linux-kernel - Re: [PATCH v2] page_alloc: Fix freeing non-compound pages

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Date:   Tue, 29 Sep 2020 23:53:07 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Nick Piggin <npiggin@...il.com>, Hugh Dickins <hughd@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] page_alloc: Fix freeing non-compound pages

On Mon, Sep 28, 2020 at 09:46:56PM -0700, Andrew Morton wrote:
> On Tue, 29 Sep 2020 02:17:19 +0100 Matthew Wilcox <willy@...radead.org> wrote:
> 
> > On Mon, Sep 28, 2020 at 06:03:07PM -0700, Andrew Morton wrote:
> > > On Sat, 26 Sep 2020 22:39:19 +0100 "Matthew Wilcox (Oracle)" <willy@...radead.org> wrote:
> > > 
> > > > Here is a very rare race which leaks memory:
> > > 
> > > Not worth a cc:stable?
> > 
> > Yes, it probably should have been.
> 
> Have you a feeling for how often this occurs?

I doubt it happens often.  I don't think I could construct a workload to
make it happen frequently.  Maybe more often with a virtualised workload
where a thread can be preempted between instructions.

> >  I just assume the stablebot will
> > pick up anything that has a Fixes: tag.
> 
> We asked them not to do that for mm/ patches.  Crazy stuff was getting
> backported.

That's a shame.  I'll try to remember to cc them explicitly in the future.

> > Although I'm now thinking of making that comment into kernel-doc and
> > turning it into advice to the caller rather than an internal note to
> > other mm developers.
> 
> hm.  But what action could the caller take?  The explanatory comment
> seems OK to me.

Use compound pages instead of non-compound pages.  Although Linus has
asked that people stop using __get_free_pages(), so maybe that will be
the direction we go in.

https://lore.kernel.org/lkml/CA+55aFwyxJ+TOpaJZnC5MPJ-25xbLAEu8iJP8zTYhmA3LXFF8Q@mail.gmail.com/